fix: invalidate index discographies after library rescan

The search index's Tier 3 (library artists) depends on MBIDs from
the artists table. If the index built before a rescan populated
those MBIDs, library artists like Flatbush Zombies wouldn't be
indexed — they're not in the sitewide top 1000 and their name
didn't match via fuzzy matching.

Fix: OnAllScansComplete hook now calls InvalidateIndexDiscographies
before StartIndexBuild. This clears the discog_built timestamp so
Tiers 2-4 re-run incrementally, picking up any new library artists
whose MBIDs were just populated by the scan.

The rebuild is incremental — only artists not already in the index
get their discographies fetched.

backend/app.go

@@ -218,6 +218,9 @@ func (yj *YellowJacketApp) OnStartup(ctx context.Context) {
 	yj.library.SetScanHooks(library.ScanHooks{
 		ResolvePhantoms: yj.playlist.ResolvePhantomTracksAfterScan,
 		OnAllScansComplete: func() {
+			// Force index Tiers 2-4 to re-check for new library
+			// artists whose MBIDs were just populated by the scan.
+			yj.explore.InvalidateIndexDiscographies()
 			yj.explore.StartIndexBuild()
 		},
 	})